The present invention relates to a dually photosensitive composition capable of exhibiting solubility behavior in the development both of the positive type and negative type depending on the conditions of exposure to light and to a method for patterning a photoresist layer formed of the photosensitive composition.
In recent electronics industry, the technology of fine patterning for the manufacture of ICs, LSIs, ultra-LSIs and the like is in continuous shift from the conventional wet process to the so-called dry process including dry etching, ion implantation, ion milling and the like techniques. These dry process techniques generally have several advantages not only in the fineness in etching and ion implantation but also in respect of the high dimensional accuracy and controllability in working and sharpness of the pattern edges in comparison with the wet process although the photoresist material used in the dry process is required to have resistance against heat and plasma and other properties in addition to the properties required for the material used in the wet process.
In a process of extremely fine working for, for example, multilayer wiring with high density of integration, on the other hand, patterning is sometimes performed on the surface of a substrate having differences in the surface level. When a photoresist layer is formed on such a surface having differences in level and exposed to light, a problem is that the optimum exposure dose may differ from portion to portion at different surface levels due to the unevenness of the surface adversely influencing the accuracy of fine working. When the photoresist material used on such a stepped surface is of the positive type, the phenomenon of so-called narrowing takes place with a smaller line width at the higher side of the step than at the lower side so that the line width cannot be uniform throughout the surface. This undesirable phenomenon can be eliminated only with extreme difficulties presenting a very serious problem to decrease the accuracy in the extremely fine working for the manufacture of ultra-LSIs and the like fine electronics devices.
As a method for solving this problem, there is proposed the multi-layer process in which the stepped surface of a substrate is provided with a relatively thick undercoating layer of an organic coating composition to have a sufficiently flat surface on which a layer of a photosensitive composition having no miscibility with the undercoating is formed. The upper layer of the photosensitive composition is first patterned by the photolithographic techniques to form a pattern-wise resist layer which serves as a masking layer in the subsequent patterning of the underlying layer of the organic coating composition by plasma etching or solvent development. This multi-layer process is in principle effective in obtaining a patterning almost as fine as desired by substantially eliminating the influences by the stepped surface condition.
A problem of this process in practice is that the patterned photoresist in the upper layer is more or less badly affected in the development of the lower organic layer to be eventually "dissolved away by the solvent for development". Therefore, proper selection of a development solvent for the organic undercoating is essential in this process but no solvents hitherto developed are quite satisfactory in this respect.
Alternatively, a modified multi-layer process is proposed in which a layer of silicon dioxide SiO.sub.2 is interposed by the techniques of vapor-phase deposition (CVD) between the organic undercoating layer and the photosensitive layer to form a triple-layer structure. Such a photosensitive material is, however, practically not advantageous because of the complicacy in the manufacturing process.
Thus, it has long been desired to develop a novel photosensitive composition suitable for use in the above described process having outstandingly high solvent resistance without necessitating the intervening layer of silicon dioxide and the like.